System and method for data reconfiguration in an optical communication network

ABSTRACT

A system and method are disclosed that may be operable to avoid upstream reassembly at an optical line terminal in a passive optical network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/893,186, filed Mar. 6, 2007, entitled “Systemand Method for Data Reconfiguration in an Optical CommunicationNetwork”, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The Gigabit Passive Optical Network (GPON) system is organized in thefollowing manner. Optical network terminals (ONTs) are located closestto the end users of the network and transmit information to and from theuser into a Passive Optical Network (PON) link. On the other side of thePON link is an optical line terminal (OLT), which aggregates the datafrom several different ONTs (typically 32 per PON link), and possiblymultiple PON links, and sends the data on into the core network.

Typically, the computers/devices connected to the ONT transmitEthernet/IP packets, meaning that the level 2 protocol is Ethernetprotocol and the level 3 protocol is Internet Protocol (IP). Therefore,the ONT ultimately receives and sends Ethernet packets. The amount ofdata an ONT can send in a given time frame is fixed by the OLT through aprocess called bandwidth allocation. In order to efficiently utilize theallocated bandwidth, the ONT fragments the Ethernet packets it receives,and wraps them into G-PON Encapsulation Mode (GEM) frames. Each GEMframe can contain a full Ethernet packet, multiple Ethernet packets,partial Ethernet packets, or any combination of the above. Therefore,the GEM frames are launched into the PON link. These frames are decodedby the OLT at the other end of the PON link, and Ethernet fragments areextracted from these GEM frames and are re-assembled at the OLT.

GEM is a method for encapsulating data over a GPON. Although any type ofdata can be encapsulated, the data types to be encapsulated depend onthe service situation. GEM provides connection-oriented communication aswell as Asynchronous Transfer Mode (ATM) communication. The concept andframing format may be similar to Generic Framing Procedure (GFP).

FIG. 1 shows how this fragmentation can induce network congestion. FIG.1 shows a PON link over which ONT1 transmits fragments of three Ethernetpackets in its time slot E1, F1, and G1, where the number represents thefragment number and the letters denote different Ethernet packets. Ingeneral, this can happen if E, F, and G belong to different trafficclasses (the OLT allocates bandwidth separately to different classes oftraffic). When these fragments are transmitted, ONT1's transmissionwindow runs out. Therefore, these fragments cannot be transmitted by theOLT into the network, and thus remain in the memory until the finalfragments of these three Ethernet packets arrive. This happens at timeT2, when ONT1 begins transmitting again and sends fragments 2 of E, F,and G, labeled E2, F2, and G2, respectively. Now, the OLT has threepackets to transmit within a relatively short window, and wastedtransmission time in the previous window, waiting for the fragments toarrive. Therefore, fragmentation and reassembly can induce congestion inan otherwise smooth stream of traffic.

A second issue is the hardware associated with reassembly. Clearly, theOLT needs to store the Ethernet fragments in a memory. When newfragments arrive, the OLT must find out whether the Ethernet fragmentsform a complete packet and, if so, pass the packet on through thenetwork. As with any buffer based solution, we need to be concernedabout buffer overflows that can result from an ONT reset/missed or lostpackets. All of these cases create a substantial implementation overheadfor the ONT. Thus, there is a need in the art for an improved system andmethod for communication in optical networks.

SUMMARY OF THE INVENTION

According to one aspect, the invention is directed to a system andmethod that will reduce upstream data congestion and simplify theimplementation of the GPON protocol in the upstream direction (thedirection corresponding to data transmission from the network users tothe carrier) Users can decompose Ethernet frames into GPON protocolunits, called GEM fragments, and transmit them upstream. One Ethernetframe can potentially span several GEM frames. GEM frames arereassembled into Ethernet frames at the OLT (optical line terminal) andtransmitted onwards, to the final destination.

Other aspects, features, advantages, etc. will become apparent to oneskilled in the art when the description of the preferred embodiments ofthe invention herein is taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purposes of illustrating the various aspects of the invention,there are shown in the drawings forms that are presently preferred, itbeing understood, however, that the invention is not limited to theprecise arrangements and instrumentalities shown.

FIG. 1 is a block diagram of a communication network;

FIG. 2 is a block diagram of a data reconfiguration engine in accordancewith an embodiment of the present invention; and

FIG. 3 is a block diagram of a computer system useable in conjunctionwith an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation, specificnumbers, materials and configurations are set forth in order to providea thorough understanding of the invention. It will be apparent, however,to one having ordinary skill in the art that the invention may bepracticed without these specific details. In some instances, well-knownfeatures may be omitted or simplified so as not to obscure the presentinvention. Furthermore, reference in the specification to phrases suchas “one embodiment” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the invention. The appearancesof phrases such as “in one embodiment” or “in an embodiment” in variousplaces in the specification do not necessarily all refer to the sameembodiment.

Gigabit passive optical networks (GPONs) are a promising last-mileoption, and have the potential to deliver high-speed services to thehome and office over optical fiber. We propose a system and method thatwill reduce upstream data congestion and simplify the implementation ofthe GPON protocol in the upstream direction (the direction correspondingto data transmission from the network users to the carrier). Users candecompose Ethernet frames into GPON protocol units, called GEMfragments, and transmit them upstream. One Ethernet frame canpotentially span several GEM frames. GEM frames are reassembled intoEthernet frames at the OLT (optical line terminal) and transmittedonwards, to the final destination. We propose a scheme in which theEthernet data extracted from the GEM fragments are each encapsulatedinto separate Ethernet frames and separate IP (Internet Protocol)fragments. Since the IP protocol supports (and indeed encourages)reassembly at the target, the IP fragments are sent into the network.

Therefore, the OLT does not have to wait for all GEM fragments to comein before sending data into the network. GEM fragments are preferablyencapsulated into IP fragments as they arrive at the OLT. This providessmooth traffic flow because the OLT does not have to wait for a completeEthernet packet to arrive before transmitting data into the network. Inaddition, upstream logic does not have to be maintained for performingreassembly. The above-described system and method preferably results ina much simpler logical implementation that avoids the need forreassembly logic.

In one embodiment, the present invention may include converting GEMfragments into valid Ethernet packets and valid IP fragments. Thesepackets and/or fragments are then transmitted upon receipt, therebypreferably eliminating the need for a reassembly buffer or complexreassembly logic. This approach also preferably avoids data trafficcongestion by more efficiently employing the available data transmissionperiod or window.

In the following section, we outline our procedure to create valid IPpackets from Ethernet fragments. As discussed above, an Ethernet packetcan span several GEM frames.

The Structure of a Gem Frame that Begins an IP Packet is Preferably asFollows:

1. GEM header a. ID b. Last/not last 2. Ethernet header 3. IP Header 4.DataSubsequent Frames Preferably have the Following Structure:

1. GEM header a. ID b. Last/not last 2. Data

From the “last/not last” indication in the GEM header, we can identifywhether a given frame is the last of a sequence or not.

The primary data structure that we maintain may be a table, indexed byGEM ID, that contains the Ethernet and IP fragments, and an entry called“Current_Frag_Offset”, which maintains the number of bytes of a given IPpacket that have been transmitted. This is beneficial for setting the“Frag_offset” field of an IP packet that identifies the exact byteposition in the IP packet at which a given fragment fits in.

Therefore, a method in accordance with one embodiment of the inventionis listed below. The invention is not limited to the specific order ofsteps listed below. Moreover, selected steps may be omitted wherebeneficial for a particular application.

1. When a GEM frame arrives, find out if it is the first frame of asequence. This can be done by simply checking the ID (every frame has aunique ID)2. If the GEM frame is the first frame of an IP Packet:

-   -   a. If a GEM frame is a first frame, recover and store the        Ethernet and IP headers in a table indexed by the ID of the GEM        frame.    -   b. Modify the GEM frame as follows:        -   i. Change the IP header to indicate that the IP packet is            fragmented.        -   ii. Change the IP packet length in the IP header to the            length of the data fragment. The data fragment's length is            computed as the difference between the GEM frame and the            point where IP data begins.        -   iii. Modify the length of the Ethernet frame to be the total            length of the Ethernet fragment embedded in the GEM frame.        -   iv. Recompute the Ethernet Cyclic Redundancy Check (CRC).        -   v. Send the packet on.

The contents and order of the above list of steps may be varied asneeded for a given application.

3. If the GEM frame is not the first frame of the IP Packet:

-   -   a. Look up the Ethernet and IP headers under the GEM frame ID.    -   b. Modify the GEM frame as follows.        -   i. The Ethernet packet length is set to length of the            Ethernet fragment embedded in the GEM packet.        -   ii. The length of the IP packet is set to the length of the            IP fragment embedded in the GEM packet.        -   iii. The IP fragment offset is set copied from the            “Current_Fragment_Offset” entry in the table.        -   iv. The Entry for “Current_Fragment_Offset” in the table is            incremented by a number equal to the length of the embedded            IP fragment.

With reference to FIG. 2, hardware for implementing the above method mayinclude a data reconfiguration engine 200 which may include a memory 220that stores the IP and Ethernet headers, and/or a CRC engine 240 thatcan recompute the CRC of the modified Ethernet fragment. Both of theseelements are relatively simple, and add up to a substantially lesscomplex solution than a full upstream reassembly engine. In addition,retransmission of data upon receipt thereof preferably provides asmoother and better regulated flow of data traffic. Data reconfigurationengine 200 may be placed in communication with the Optical LineTerminal, or OLT, of FIG. 1. Alternatively, engine, or processor, 200may be placed in communication with one or more other components withinan optical communication network (in addition to, or in place of theOLT). Data reconfiguration engine 200 may be any suitable computingdevice having one or more processors, one or more data storage devices,and/or one or more communication links internal to engine 200 and todevices external to engine 200 for conducting data communicationtherewith.

Above, we present a technique to improve traffic flow and simplifyhardware implementation of an upstream GPON link. Our technique is basedon the observation that fragmentation and reassembly are (i) complex toimplement, and (ii) can lead to holes and bursts in transmission,thereby creating turbulent and potentially congested traffic flow, asopposed to the smooth flow desired for the GPON protocol. Therefore, wepropose a scheme that creates valid IP/Ethernet packets from GEMfragments and that enables data to be retransmitted without asignificant need for data buffering and without significant delay, asexperienced in the prior art. The disclosed system and method preferablyavoids potential congestion issues and is also simple to implement.

FIG. 3 is a block diagram of a computing system 300 adaptable for usewith one or more embodiments of the present invention. In one or moreembodiments, central processing unit (CPU) 302 may be coupled to bus304. In addition, bus 304 may be coupled to random access memory (RAM)306, read only memory (ROM) 308, input/output (I/O) adapter 310,communications adapter 322, user interface adapter 306, and displayadapter 318.

In one or more embodiments, RAM 306 and/or ROM 308 may hold user data,system data, and/or programs. I/O adapter 310 may connect storagedevices, such as hard drive 312, a CD-ROM (not shown), or other massstorage device to computing system 300. Communications adapter 322 maycouple computing system 300 to a local, wide-area, or Internet network324. User interface adapter 316 may couple user input devices, such askeyboard 326 and/or pointing device 314, to computing system 300.Moreover, display adapter 318 may be driven by CPU 302 to control thedisplay on display device 320. CPU 302 may be any general purpose CPU.

It is noted that the methods and apparatus described thus far and/ordescribed later in this document may be achieved utilizing any of theknown technologies, such as standard digital circuitry, analogcircuitry, any of the known processors that are operable to executesoftware and/or firmware programs, programmable digital devices orsystems, programmable array logic devices, or any combination of theabove. One or more embodiments of the invention may also be embodied ina software program for storage in a suitable storage medium andexecution by a processing unit.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A device substantially as shown and described.